Final answer:
Endorphins are peptide secretions from neurons that can act both as hormones when released into the bloodstream and as neurotransmitters. They play a role in pain reduction and pleasure and are sometimes called the body's natural painkillers. Insulin and cortisol are hormones but do not act as neurotransmitters.
Step-by-step explanation:
Peptide secretions from neurons that are secreted directly into the bloodstream and also act on other neurons as neurotransmitters are generally known as neuropeptides. Among the options given, the correct answer is endorphins. These peptides have the dual function of behaving like hormones when they are released into the bloodstream and acting upon distant target cells, as well as serving as neurotransmitters when they act on neighboring neurons.
Endorphins are produced in the central nervous system and the pituitary gland. They are known for their role in pain reduction and pleasure, sometimes referred to as the body's natural painkillers. It's important to note that insulin is also a peptide hormone secreted by beta cells in the pancreas, primarily functioning to lower blood glucose levels. The secretion of insulin can be stimulated by a hormone called glucose-dependent insulinotropic peptide. However, insulin does not fit the criteria of the question, as it does not act as a neurotransmitter.
Cortisol, another option, is a steroid hormone released by the adrenal glands in response to stress and a low level of blood glucose. While cortisol does affect several systems in the body, including the brain, it is not typically classified as a neurotransmitter.